Inverse feedback circuits



Feb. 23, 1943.

M. CROSBY INVERSE FEEDBACK CIRCUIT File d Dec. 16, 194; 2 Sheets-Sheet lTORY $301733 ATTOR EY INVVEN Q 5 "WWW ORE} k N Feb. 23, 1943. M. G;CROSBY INVERSE FEEDBACK CIRCUIT Filed De c. 16, 1941 2 Sheets-Sheet 2 MA MN ab m2; W

INVENTO A T1:ORNEY Patented Feb. 23, 1943 IN VERSE FEEDBACK CIRCUITSMurray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application December 16, 1941, SerialNo. 423,162

'7 Claims.

My present invention relates to inverse audio feedback circuits adaptedfor use in amplitude modulated carrier Wave receivers.

In the past it has been the usual practice to apply audio feedbackpotentials to an audio amplifier input, the sense of feedback beingdegenerative thereby to reduce distortion arising within the amplifier.For example, H. S. Black has described such inverse feedback circuits inan article entitled Stabilized feedback amplifiers published in theJanuary 1934 issue of the Bell System Technical Journal. I have foundthat inverse feedback of audio potentials into the detector circuit of aradio receiver serves to reduce distortion encountered in the detectorcircuit. By

"the process disclosed herein an audio voltage is introduced into thedetector circuit which opposes the audio output voltage in a manner toreduce the effective percentage of modulation of the received carrierwaves. Since the percentage of modulation in the detector is reducedmaximum modulation capabilities of the detector are not attained, anddistortion encountered by virtue of high percentage modulation isreduced.

Accordingly, it may be stated that it is one of the main objects of mypresent invention to provide an amplitude modulated carrier wavereceiving system in which inverse audio feedback is utilized in such amanner as to reduce distortion encountered in the detector circuit.

Another important object of the invention is to reduce the percentage ofmodulation of the carrier waves applied to the detector by applying theaudio feedback potentials as modulation on one of the amplifierspreceding the detector whereby the feedback voltage modulates, or it canequally be said unmodulates, the incoming modulated wave energy so thatthe percentage modulation of the carrier waves fed to the detector isreduced.

Another object of my invention is to provide inverse audio feedback to aplurality of demodulator devices used in a diversity receiving systemthereby efiectively to minimize distortion arising by virtue ofselective fading of the type wherein the carrier fades with respect tothe modulation side bands.

Still other objects of my invention are generally to improve inverseaudio feedback circuits, and more particularly to provide inversefeedback circuits which are reliable in operation and economicallyassembled.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

n the drawings:

Fig. 1 shows one embodiment of the invention,

Fig. 2 shows a modification of the invention,

Fig. 3 shows a diversity receiving system embodying the invention.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,there is shown in Fig. 1 that portion of an amplitude modulated carrierwave receiving system which is involved. in the present invention.Merely by way of illustration, let it be assumed that the demodulatordiode 4 is the second detector of a superheterodyne receiver. In suchcase the intermediate frequency (I. F.) transformer feeding the diode 4has its primary and secondary resonant circuits l and 2 each tuned tothe operating I. F. value. The primary circuit I would, of course, becoupled to the output electrodes of the last I. F. amplifier tube.

Those skilled in the art are fully aware of the construction of thetypical superheterodyne circuits preceding the primary circuit I, and,therefore, they need not be shown in detail. In general, the signalcollector may collect signals in the broadcast range of 550 to 1700kilocycles (kc.), or it may collect signals in the short wave ranges ifsuch be the case. One or more stages of radio frequency amplificationmay be utilized to feed the usual first detector, or converter. Theresulting I. F. energy, which may have any suitable frequency value, isfed to one or more stages of I. F. amplification. The demodulator, orsecond detector, circuit consists of the diode 4 whose anode isconnected to the high potential side of the input circuit 2, while thelow potential side of the latter is connected to ground through the loadresistor 5 which is by-passed for I. F. carrier currents by condenser 6.a

The radio frequency by-pass condenser 9 connects the cathode of diode 4to ground. It will be recognized that the demodulator circuit is of theusual and well known type. Audio, or modulation frequency, voltagedeveloped across resistor 5 is transmitted to the control grid of thefirst audio frequency amplifier ll through a path which includes adirect current blocking condenser l. The latter is arranged in serieswith the potentiometer resistor 8 Whose lower end is audio amplifier II.

The inverse, or degenerative, audio feedback a path comprises lead I 6connected between the upper end of the damping resistor I 4 and thecathode of diode 4. The feedback path includes the radio frequency chokecoil I0. It will, therefore, be seen that the audio voltage is inverselyfed back directly into the diode detector circuit. In the operation ofthis circuit the audio feedback potentials are. degeneratively intro.-duced into the cathode circuit of the detector diode so that thepercentage of modulation pres ent on the carrier wave applied to thedetector is effectively reduced.

This action would be similar to the type of circuit where the feedbackvoltage was fed directly to the grid or cathode circuit of tube II,except for the fact that by introducing it as shown the distortionintroduced by the diode is reduced as well as that introduced by the Itis well known that the ordinary diode circuit introduces distortion whendetecting a modulated wave with a high degree of modulation.Consequently, this circuit of Fig. 1, which reduces the efiectivepercentage of modulation present on the carrier wave in the detectorcircuit, will also reduce the distortion encountered in that circuit.

In Fig. 2, I have shown a modification of the invention wherein theaudio feedback potential is utilized to unmodulate the incomingamplitude modulated carrier wave. In this case the diode demodulator 4has the primary circuit I of I. F. transformer 3 arranged in the platecircuit of an I. F. amplifier tube I02 which may be of the pentagridtype. Control grid IOI of this tube may be connected to the secondaryresonant circuit of the I. F. transformer 3', the primary circuit beingarranged in the output circuit of a prior I. F. amplifier. The thirdgrid I9! of tube I02 is connected to the degenerative feedback path I6which includes the radio frequency choke coil I0. The remainingelectrodes of tube I02 are conventional, and their functions are wellunderstood. The cathode of tube I02 may be connected to ground throughthe usual self-biasing resistor I02 which is by-passed for I. F.currents.

In this circuit arrangement the audio voltage developed across resistor5 is transmitted to the same type of audio frequency amplifier networkas shown in Fi 1. It will be understood that the degenerative feedbacklead I6 is connected in the same manner as in Fig. 1; that is'to say, itwould be connected to any desired point along the audio amplifiernetwork. Automatic volume control (A. V. C.) may be provided in theusual and well known manner by connecting the A. V. C. bias connectionto the anode end of resistor 5. The A. V. C. lead would, of course,include a proper pulsating voltage filter 5. The A. V. C. bias. may beapplied to any of the amplifier control grids between the signalcollector and tube I02.

The operation of the circuit in Fig. 2 depends upon the fact that theaudio feedback voltage fed to grid IEII applies a modulation voltage tothat grid in such a phase as to reduce the effective percentage ofmodulation of the modulated carrier waves fed through transformer 3 tocontrol grid IN. The phase of the modulation voltage on grid IOI is suchas to unmodulate the applied modulated carrier wave. It will, of course,be understood that the modulation voltage applied to grid IOI' will notbe of sufiicient magnitude completely to remove the modulation on thecarrier wave at transformer 3. The magnitude of unmodulated voltage issufficient so that the modulated carrier wave at transformer 3 has asubstantially reduced percentage of modulation. This reduced percentagemodulation wave is then fed .to the diode detector 4, and produces lessdistortion in the detector circuit by virtue of the reduced percentagemodulation.

The present invention has a particular advantage in the case of adiversity receiver system in such cases where there is encounteredselective fading of the type wherein the carrier fades with respect tothe modulation side bands. In Fig. 3 there is shown in highly schematicform a diversity reception system. Since those skilled in the art arefully acquainted with the construction of the diversity receptionsystems and the functioning thereof, it is believed that the schematicshowing is fully warranted. Briefly, numerals 203, 204 and 205 designatethree independent receiving systems, each of which may be of thesuperheterodyne type. The grounded antenna circuits 200, MI and 202provide modulated carrier pick-up devices for receivers 203. 204 and 205respectively.

As is well known, the signal pick-up devices of the respective receiversare geographically so related as to secure the benefits of diversityreception. The diode demodulator 209 is fed with the I. F. energy fromreceiver I through the I. F. transformer 20%. Diode demodulator H0 isfed from receiver '2 through I. F. transformer 201. Diode ZII is fedwith I. F. energy through the I. F. transformer 20%. It will beunderstood that each of I. F. transformers 206, 201 and 208 have theirrespective primary and secondary resonant circuits tuned tosubstantially the same oper ating I. P. value.

The low potential sides of the secondary circuits of each of the I. F.transformers are connected in common to ground through a common loadresistor 213 which is shunted by the I. F. by-pass condenser 2E2. Thecathodes of diodes 209, 2H 2H are connected in common, and the commonconnection is connected to ground through the radio frequency by-passcondenser EN. The audio voltage developed across load resistor H3 is fedthrough the direct current blocking condenser 2I-I to the control gridof the audio amplifier 2 I 9. The audio potentiometer 2I8 has itsresistor in series with blocking condenser 2H. Here, again, there may beutilized one or more audio amplifier stages between the audio amplifier2I9 and the final audio amplifier 220. The audio transformer 22I mayhave its secondary winding shunted by the clamping resistor 223, and thejack 222 may be used to feed any audio utilization means from across thesec ondary winding of transformer 22I.

The degenerative feedback path includes the radio frequency choke coil2I5. The degeneration voltage is fed from the high potential side of thesecondary winding of transformer 22l to the common cathode connection ofthe various diodes, the feedback path including the radio frequencychoke coil 215. Numeral 216 designates the A. V. C. connections to thevarious receiver amplifier circuits from the anode end of the loadresistor 2|3. The manner of making the A. V. C. connections to thevarious controlled stages of the receivers is so well known that theschematic representation is believed to be sufficient. The function ofthe A. V. C. circuit, of course, is to maintain the carrier amplitude ateach of the demodulator input circuits as constant as possible in spiteof the fading occurring at the signal collector devices.

It will be observed in the arrangement of Fig. 3 that the inverse audiofeedback is applied to the common cathode circuit of the demodulators209, 2l0, 2| l. Hence, it can be seen that the inverse feedback tends toreduce the percentage of modulation existing in the demodulatorsregardless of which receiver is contributing the predominant amount ofenergy. If a single receiver is predominating, the feedback operates toreduce the effective percentage of modulation so that distortion due tofading of the carrier on that receiver is minimized. If more than onereceiver is contributing to the output, the feedback automaticallyreduces the effective percentage of modulation produced by the resultantof the outputs of the several receivers. It will, therefore, be seenthat selective fading occurring in shortwave communication can beeffectively reduced by virtue of the diversity reception augmented bythe inverse audio feedback to each of the demodulators of the separatereecivers. It is to be understood that the arrangement shown in Fig. 2could also be utilized in connection with the diversity reception systemshown in Fig. 3.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a receiving system for modulated carrier waves subject toselective fading, said receiving system comprising a plurality ofindependent modulated carrier wave transmission networks each includinga separate demodulator, a common load element for said demodulators fordeveloping modulation voltage, means for utilizing said modulationvoltage, and a degenerative modulation voltage feedback path betweensaid utilization means and said demodulators for minimizing the effectsof said selective fading.

2. In a diversity reception system of the type comprising a plurality ofseparate modulated carrier Wave receivers each provided with anindependent signal collector device, each of said receivers having anindependent demodulator, a common output circuit for said demodulatorsfor developing modulation voltage, a modulation voltage network coupledto said common output circuit, and an inverse modulation voltage feedback path between said utilization network and said common outputcircuit for substantially reducing the effects of selective fading.

3. In a diversity reception system of the type comprising a plurality ofmodulated carrier wave receivers each provided with a signal collectordevice, each of said receivers having a respective demodulator, a commonoutput circuit for said demodulators for developing modulation voltage,an inverse modulation voltage feedback path connected to said commonoutput circuit for substantially reducing the effects of selectivefading, each of said demodulators being of the diode type, and therebeing a common cathode connection between said common output circuit andthe cathodes of said diodes, said feedback path being connected to saidcommon cathode connection.

4. In a diversity system of the type comprising a plurality of separatemodulated carrier wave receivers connected to provide a single detectedoutput, an inverse modulation feedback path between said single detectedoutput and each of said separate modulated wave receivers forsubstantially reducing the effective percentage of modulation of theenergy received from said separate receivers.

5. In a reception system of the type comprising separate modulatedcarrier wave receivers, each of said receivers having an independentdemodulator, an output circuit for said demodulators for developingmodulation voltage, a modulation voltage utilizing network coupled tosaid common output circuit, a modulation voltage feedback path betweensaid utilization network and said common output circuit forsubstantially reducing the effects of selective fading, each of saiddemodulators being of the diode type, and there being a common cathodeconnection between said common output circuit and the oathodes of saiddiodes, said feedback path being connected to said common cathodeconnection.

6. In a diversity reception system of the type comprising a plurality ofseparate modulated carrier wave receivers each provided with anindependent signal collector device, each of said receivers having adiode demodulator, a common output circuit for said diodes fordeveloping modulation voltage, a modulation voltage amplifier networkcoupled to said common output circuit, and an inverse modulation voltagefeedback path between said amplifier network and said common outputcircuit for substantially reducing the effects of selective fading.

7. In a diversity reception system of the type comprising a plurality ofseparate modulated carrier wave receivers each provided with a signalcollector device, each of said receivers having an independent diodedemodulator, a common output circuit for said demodulators fordeveloping modulation voltage, a modulation voltage amplifier networkcoupled to said common output circuit, an inverse modulation voltagefeedback path between said utilization network and said common outputcircuit for substantially reducing the effects of selective fading, acommon cathode connection between said common output circuit and thecathodes of said diodes, said feedback path being connected to saidcommon cathode connection.

MURRAY G. CROSBY.

